The invention relates to a method of thermal disposal of all types of sewage sludge and to an installation for carrying out this process.
Presently, in the Federal Republic of Germany approximately 50 Million tons of sewage sludge are produced yearly during waste water purification. This value refers to sewage sludge containing 5% dry solids and 95% water. The quantity of dry solids consequently adds up to about 2.5 million tons/year. Fifty percent of these dry solids come from domestic and industrial sewage sludge, respectively.
Until about 10 years ago, sewage sludge was disposed of without problems by distributing it onto farm land and forest areas. Because of its high contents of potash, nitrogen and phosphor it used to be a valuable and also economic fertilizer. The content of heavy metals, which frequently was considerably high, as well as the fear that sewage sludge depending on its origin could be contaminated with polychlorinated biphenyl and dioxines has brought sewage sludge as fertilizer into discredit and prevented its distribution.
With these rising problems and at the culmination of the oil crisis at the end of the 1970's, one began to search for new ways to use sewage sludge energetically by incineration. Simultaneously, controlled dumping was used.
Against the background of these attempts, disposal of sewage sludge actually is carried out as follows:
59% is dumped,
29% is used in agriculture and forestry,
9% is composted.
Lately it becomes more and more difficult and expensive to dump sewage sludge in the neighbourhood of densely populated areas. Appropriate dump areas are scarce. Therefore, the percentage of incineration is increasing rapidly. But also against incinceration opposition begins to form because of the aforementioned contamination of flue gases with dioxines and other chemical compounds suspected to be carcinogenic.
The disadvantage of the incineration methods developed thus far consists in that the ashes, and above all the dust from filtration, are considered as special waste because of its possible contamination with dubious substances such as heavy metals, and because of a carry-over of halogenated cyclic hydrocarbon deriving from an incomplete incineration, which entails problems of a special waste dumping or disposal.
All methods thus far developed for thermal disposal of sewage sludge include the following process steps:
thickening;
dewatering (dehydration);
drying;
incineration;
treatment of gases
disposal of residuals.
For each process step different reliable components are available.
For example, dewatering is carried out with filter presses, belt filter presses or centrifuges after preceding flocculation and thickening.
Drying uses cylindrical rotary kilns, kilns with overlying beds, fluidized bed furnaces, grinding driers or indirect driers of different constructions. The incineration is carried out either in combination with drying directly in kilns with overlying beds or separately in following fluidized bed furnaces, dust ovens or in slag tap furnaces. The flue gases are post-combusted in electric filters and washers, if necessary.